This invention relates to a display device with which an observer can see images or characters superimposed on a background view, and which may be used for a car, a ship or other vehicles, or a measurement instrument.
Such a display device having a transparent property is disclosed in Japanese laid open patent application (Tokukaihei) 7-96771 or 7-315075, for example. This display device enables an observer, e.g., a driver, to see information images or characters without turning his or her eyes away from a view that can be seen through a windshield. This type of display, which is called a "heads up display", has a following configuration.
A transparent type LCD (Liquid Crystal Display) on which a certain information image is displayed is lighted by a light system including a halogen lamp and a condenser lens. The obtained light ray containing the displayed image is reflected by plural mirrors sequentially and finally by a semitransparent mirror to reach eyes of the observer. The semitransparent mirror is disposed in a windshield of the car, or on the surface of the windshield, or close to the windshield. Therefore, the information displayed on the LCD is observed as a virtual image behind the semitransparent mirror by the observer. In other words, the observer can see the information superimposed on the outside view that can be seen through a windshield.
However, the above-mentioned display in the prior art has some disadvantages. It is difficult to see the image of the information in the bright external light of a sunny day. To keep good visibility for the information image, a high intensity light source is required for the light system. As a result, power consumption increases, and a space and/or device for cooling the light source is necessary.
Moreover, in the above-mentioned display, the transparent type LCD, halogen lamp and other members for forming images are disposed distantly from the windshield. The displayed image has to travel by reflecting on the plural mirrors from the LCD to the neighborhood of the windshield. Therefore, the contour of the whole display apparatus is large, so that a large space for housing the apparatus is required in a dashboard, for example. Additionally, the visibility of the display is not good since the intensity of the displayed image is not uniform due to unevenness of lighting that is intrinsic to the light system. To improve the uniformity of the intensity the light system must become larger and/or more complicated.